Oil production by gas drive from adjacent strata



F'IPTEME NOV. 4, 1969 CORNELlUs 3,476,185

OIL PRODUCTION BY GAS DRIVE FROM ADJACENT STRATA Filed Nov. 14, 1967 OIL.

INVENTOR.

J Wm A T TORNEYS United States Patent 3,476,185 OIL PRODUCTION BY GAS DRIVE FROM ADJACENT STRATA Archie J. Cornelius, Bartlesville, Okla., assignor to Phillips Petroleum Company, a corporation of Delaware Filed Nov. 14, 1967, Ser. No. 682,774 Int. Cl. E211) 43/20, 43/12 US. Cl. 166-274 7 Claims ABSTRACT OF THE DISCLOSURE In many low-gravity oil reservoirs, selective water encroachment results in watering out the reservoir when only 2-8 percent of the oil in-place has been produced by primary production methods. Pressure maintenance in such a situation by water injection merely aggravates the problem. One method of overcoming the problem to a considerable extent comprises maintaining the pressure by gas injection at the crest of the structure. The gas does not exhibit the extreme fingering tendency experienced by waterfloods, perhaps as a result of gravity segregation, in situ emulsion formation, or perhaps by some unexplained mechanism. Maintaining the reservoir pressure reduces or eliminates the tendency for water to encroach on the petroleum stratum from adjacent aquifers.

This invention is concerned with a method of oil production which substantially overcomes the foregoing problem.

Accordingly, it is an object of the invention to provide an improved process for producing oil from an oil stratum with substantially increased production. Another object is to provide an oil-production method which takes advantage of or makes use of a gas formation lying either above or below an oil stratum with an impermeable layer lying between the tWo strata. A further object is to provide a process for producing oil from an oil stratum lying adjacent an aquifer while preventing encroachment of water on the oil stratum. Other objects of theinvention will become apparent to one skilled in the art upon consideration of the accompanying disclosure.

Oil strata containing heavy oil are frequently found lying above or below a formation containing natural gas with an impermeable layer such as shale separating the two strata. In situations of this nature, the oil wells are completed only in the oil sands and not in the gas sands, as a general practice. The present invention is applicable to structures of this nature wherein adjacent oil and gas sands are separated by an impervious structure.

A broad aspect of the invention, as applied to adjacent oil and gas sands, separated by an impervious layer, comprises injecting an aqueous flood, such as water free of or containing additives, into the gas formation to force gas therefrom into a well communicating with the oil stratum so as to force the gas into the oil stratum and drive oil into an offset-production well penetrating the oil stratum. A first conduit means is provided communieating with the gas formation but closed to the oil stratum, a second conduit means offset from the first conduit means is provided communicating with the gas formation and with the oil stratum, a third conduit means offset from the second conduit means is provided communicat- 3,476,185 Patented Nov. 4, 1969 ing only with the oil stratum, and water or aqueous fluid containing conventional additives, if desired, is injected through the first conduit into the gas formation to force gas therefrom into the second conduit and from there into the oil stratum to build up stratum pressure thereby effecting gas drive of the oil into the third conduit and reducing any possible water encroachment from an adjacent aquifer.

A more complete understanding of the invention may be had by reference to the accompanying schematic drawing of which FIGURE 1 is an elevation through an earth structure containing a gas formation lying above an oil stratum with conduits for effecting the process of the invention; and FIGURE 2 is a similar view with a different arrangement of strata and conduits.

Referring to FIGURE 1, an oil formation 10 and a gas stratum 12, separated by an impermeable shale layer 14 is penetrated by Wells 16 and 18. Well 20 penetrates gas formation 12 and is provided with casing 22 which is perforated within gas sand 12 as at 24. Well 16 is provided with casing 26 which contains perforations 28 in gas sand 12 and perforations 30 in oil sand 10. Similarly, well 18 is provided with casing 32 which is perforated only in oil sand 10 by perforations 34. Well 18 is also provided with a tubing string 36 including a pump 38 on the lower end thereof for producing oil in conventional manner. Strata 10 and 12 slope slightly downwardly to the right and upwardly to the left toward the crest of the structure. In this arrangement, it is advantageous to position well 16, communicating with both strata, near the crest of the structure and production well 18 and waterinjection well 20 in the lower side of the structure so that oil flows downwardly toward well 18. Also, with this arrangement, water in aquifer 40 is more readily prevented from encroaching the oil stratum.

In operation with the arrangement illustrated in FIG- URE 1, water is injected into well 20 through valved water line 42 and through perforations 24 in casing 22 under sufficient pressure to force gas from formation 12 into well 16 through perforations 28. Water injection is continued so as to build up the gas pressure in well 16 and cause the gas to flow through perforations 30 into oil stratum 10 thereby pressuring stratum 10 and producing oil in well 18 from which it is forced or pumped through tubing string 36 to an oil production line above ground, not shown. The technique of injecting water into the gas formation has the effect of building up the gas pressure in stratum to produce oil into well 18 and also to prevent ingress of water from the aquifer into the oil stratum during this production phase of the operation. At the stage of the process illustrated in FIGURE 1, gas has been forced into the oil stratum through perforations 30 to occupy a substantial area surrounding well 16 extending generally to line 44.

Referring to FIGURE 2, oil stratum 10 lies above gas formation 12 and these strata are penetrated by wells 16 and 46, well 16 being the equivalent of well 16 in FIG- URE 1 and well 46 functioning as both wells 18 and 20 of FIGURE 1. Well 46 is provided with a casing 48 which is perforated within the oil sand by perforations 50 and within the gas sand by perforations 52, A tubing string 54 extends to a pump 56 within oil sand 10 and a water line 58 connects with a water conduit 60 extending from the well head through a packer 62 into gas formation 12. This arrangement permits injection of water from line 58 through conduit 60 into the gas formation via perforations 52 while preventing invasion of water through perforations 50 into oil stratum 10. The gas driven from formation 12 through perforations 30 in casing 16 enters oil stratum 10 via perforations 28 and forces oil into well 46 above packer 62 from which 3 it is produced by means of pump 56 through tubing string 54 for delivery to a production line.

It should be obvious that three separate wells can b utilized with the structure illustrated in FIGURE 2 and that the structure illustrated in FIGURE 1 can be pro duced in accordance with the invention by utilizing a twowell arrangement similar to that shown in FIGURE 2 with pump 56 positioned within the lower oil stratum 10 and tubing string 54 extending through packer 62. This arrangement would eliminate the need for water conduit 60.

The process described has the effect of establishing gas cap pressure maintenance in the oil sand without requiring expensive compressor facilities and only a minimum amount of water-handling equipment such as is conventionally available in oil field operations. The process of the invention increases the ultimate oil production and the daily oil-production rate while reducing production costs due to decreasing water production. The process enjoys all of the advantages of conventional gas pressure maintenance without requiring gas compressors for those structures with a separate gas sand to which the process is applicable.

If desired, the water injection through line 42 or line 58 can be followed by an aindrive.

The following example illustrates the invention but is not to be construed as unnecessarily limiting it.

' lenses most of which exhibit lateral continuity and competent shale interlayers between the individual sand lenses. The upper sand contains gas in certain areas of the field, and is separated from the next lower sand by a competent shale barrier. Total oil production from the field is on the order of 30,000 b.o.p.d. with an increasing water cut of about 40 percent at the present time. The water production is believed due to water encroachment from an adjacent equifer connected to part of the lower sand lenses. Lowering of reservoir pressure by production encourages the water to finger through the viscous oil and will eventually result in watering out the field with only about 5-7 percent of the oil in place recovered.

To arrest the reservoir pressure decline and subsequent water encroachment, it is necessary to maintain a volumetric balance, i.e., to inject at least as much fluid into the reservoir as is withdrawn. At the present time this requires injecting the 12,000 b.p.d. of water produced plus an additional 30 ,000 b.w.p.d. to replace the produced oil. The Water is injected at the edges of the upper gas sand through wells perforated only into this interval. About 2-3 wells are required to dispose of the 42,000 b.w.p.d. with wellhead pressures limited to 1000 p.s.i. and sand-face injection pressures of 2200 p.s.i. Two-three wells are in the crest of the gas sand with perforations in the gas sand and in the lower oil producing sands, the gas sand is connected to the oil reservoir. The injected water drives the gas from the gas sand into the oil sand to form a gas drive for pressure maintenance purposes. It has been found that the gas drive provides an eflicient method of maintaining reservoir pressure in heavy oil reservoirs and doubles the amount of oil which may be produced by primary methods. In the reservoir described, with 1.1 billion stock tank barrels of oil in place, increasing the primary recovery by 5 percent of the oil in place results in additional production of 55 million barrels of oil.

Certain modifications of the invention will become apparent to those skilled in the art and the illustrative details disclosed are not to be construed as imposing unnecessary limitations on the invention.

I claim:

1. A process for producing oil from an underground oil stratum lying adjacent but separated from an underground gas formation by an impermeable underground barrier between said oil stratum and said gas formation,

which comprises the steps of:

(1) providing first conduit means communicating with said gas formation but closed to said oil stratum;

(2) providing second conduit means, offset from said first conduit means of step (1), communicating with both said gas formation and said oil stratum;

(3) providing third conduit means, offset from said second conduit means of step (2), open only to said oil stratum;

(4) injecting aqueous fluid into said gas formation through said first conduit means of step (1) so as to drive gas from said gas formation into and through said second conduit means of step (2) and into said oil stratum without bringing said gas to the surface, said gas driving oil from said oil stratum into said third conduit means of step (3); and

(5) recovering the produced oil from said third conduit means of step (3).

2. The process of claim 1 wherein said first and third conduit means are in' separate wells.

3. The process of claim 1 wherein said first and third conduits are in the same well.

4. The process of claim 1 wherein the aqueous flood drive of step (4) is followed by an air drive.

5. The process of claim 1 wherein said gas formation lies above said oil stratum.

6. The process of claim 5 wherein said gas formation and said oil stratum are inclined and said first conduit means penetrates the top of said gas formation at a level substantially below the level at which said second conduit means penetrates the top of said gas formation.

7. The process of claim 1 wherein said gas formation lies below said oil stratum.

References Cited UNITED STATES PATENTS 2,192,236 3/1940 Nalley 1669 2,371,840 3/1945 Otis.

2,584,605 2/1952 Merriam et al. 166-11 2,842,204 7/ 1958 Homer 1669 2,856,000 10/1958 Barron l669 134,434 5/1964 Wooddy 166-9 215,198 11/1965 Willman 1669 3,354,952 ll/1967 Engle 16610 X CHARLES E. OCONNELL, Primary Examiner I. A. CALVERT, Assistant Examiner 

